Adenovirus vectors (Ad) are the second largest group of viral vectors that are extensively used in clinical trials in the US. The interest in Ad has recently expanded due to its potential as a vector for vaccination against anthrax and other life threatening infection agents. Despite significant knowledge regarding Ad interactions with cells in vitro, the molecular mechanisms governing infectivity, biodistribution and toxicity of systemically applied Ad vectors remain poorly understood. We hypothesize that the prevention of liver-mediated Ad clearance from the blood should allow for the improvement of Ad persistence in the circulation upon systemic vector application. We also hypothesized that combining in one vector the beneficial capsid modifications, which provide both vector targeting and stability in the blood, represents a feasible approach for the development of novel safe and effective therapeutics based on Ad. Recently we discovered a novel coagulation factor IX/blood factor-dependent pathway, responsible for the trapping of adenovirus vector by hepatocytes and Kupffer cells in vivo. This discovery represents a basis for the current proposal with the following specific aims: 1. Identify amino acid residues within Ad5 and Ad35 fiber knob domains responsible for binding to FIX. 2. Develop Ad5-based vectors possessing Ad5 and/or Ad35 fibers ablated for their binding to FIX and analyze their bio-distribution and persistence in circulation in a mouse model. 3. Analyze efficiency of target cell transduction by lung carcinoma cell-targeted, FIX binding-ablated Ad, upon its systemic application in a mouse model. These studies will dramatically improve our understanding of the mechanisms of Ad-host interactions in vivo and may ultimately lead to the development of safe and efficient Ad vectors for the therapy of a wide range of inborn and acquired human diseases.